High strength steel



United States Patent 3,181,945 HIGH STRENGTH STEEL Arthur R. Elsea,Columbus, Ohio, assignor, by mesne assignments, to The BattelleDevelopment Corporation, Columbus, Ohio, a corporation of Delaware NoDrawing. Filed Oct. 5, 1961, Ser. No. 143,066 6 Claims. (Cl. 75-423)This invention relates to ultrahigh-strength steel com positions, and,more particularly, to alloys exhibiting 0.2 percent offset yieldstrengths of 290,000 p.s.i., or greater, combined with good ductility.

In the design of solid-propellant rocket motors, one of the mostcritical problems is to find suitable ultrahigh-strength sheet materialsfrom which fuel containers can be fabricated. It is desirable to useultrahighstrength materials for this application so as to reduce theweight of the fuel containers. However, in the past, as the strengthlevel of available engineering materials increased, the ductilitydecreased, and the sensitivity to notches and other inadvertent stressraisers increased. Furthermore, it is characteristic of all structuralmaterials that they exhibit less ductility in the presence of biaxial ortriaxial tensile stresses than they do in the presence of uniaxialtensile stresses. Service loads imposed on solid-propellant fuelcontainers result in fairly severe biaxial tensile stresses, and anytype of stress raisers (such as small flaws from the fabricationprocess, or possibly nonmetallic inclusions or microcnacks in themetallic structure) may lead to catastrophic brittle failures at nominalstresses far below the expected strength of the material as determinedby uniaxial load.

It is, therefore, an object of this invention to provide alloyspossessing ultrahigh tensile strengths with good ductility. It is afurther object of this invention to provide steel alloy materialspossessed of ultrahigh strengths together with high fracture strengthsin the presence of biaxial and triaxial tensile stress, and in thepressure of small flaws. Other objects and advantages of the presentinvention will be apparent from the following detailed descriptionthereof.

In general, this invention comprises alloy steel compositions containingabout 0.45 to about 0.60 carbon, about .40 to about 1.25 manganese,about 1.8 to about 2.25 silicon, about 1.2 to about 1.8 nickel, andabout 0.16 to about 0.35 vanadium, balance iron. For some applications,the present alloys may also include boron in agiounts ranging from about0.0003 to about 0.01. A preferred composition comprises about 0.5carbon, about 1.0 manganese, about 2.0 silicon, about 1.5 nickel, about0.30 vanadium, and, for some applications, about 0.01 boron, balanceiron. (All percentages are weight percentages.)

EXAMPLE 1 An alloy was prepared having a nominal composition of 0.50 C,1.0 Mn, 2.0 Si, 1.5 Ni and 0.3 V, balance Fe. The tensile strength ofthis alloy was found to be 335,000 p.s.i., and the 0.2 percent offsetyield strength was 300,000 p.s.i. The alloy exhibited an elongation of 8percent in 2 inches and a reduction of area of 29 percent.

EXAMPLE 2 An alloy having the composition of the alloy of Example 1 plus.01 percent B was prepared. This alloy had a tensile strength of 325,000p.s.i. and a 0.2 percent oifset yield strength of 290,000 p.s.i.However, the elongation increased to 9 percent and the reduction in areaincreased to 32 percent as compared with values of 8 percent and 29percent respectively for the alloy composition of Example 1.

It will be seen from Examples 1 and 2 above that the addition of a smallamount of boron has little efliect on the strength of the presentalloys, but improves the ductility by a significant amount. This effecthas been studied in more detail and is illustrated by the data in TableI which show the effect of additions of .0003 to 0.01 percent boron.

Table I.Efiect of boron on mechanical properties of the NiV steel[Refrigerated 125 1 tempered 600 F.]

Tensile 0.2 per- Elonga- Reduc- Steel Boron strength, cent yield tion in2 tion in designation added, 1,000 strength, inches, area,

percent p.s.i. 1,000 percent percent p.s.i.

AUSTENITIZED AT 1,700 F.

AUSTENITIZED AT 1,650 F.

AUSTENITIZED AT 1,550 F.

1 Results obtained with 0.505-inch-diameter specimens.

The data in Table I above support the conclusion that boron additions,in amounts ranging from 0.0003 to 0.01 percent, improve ductility of thehigh strength alloys without substantially adversely affecting thestrength properties.

EXAMPLE 3 An alloy having the following composition was prepared: 0.49C, 1.01 Mn, 2.12 Si, 1.53 Ni, 0.30 V. balance essentially Fe. Transverseand longitudinal properties of this alloy were determined usingspecimens that were austenitized at 1700 F., oil quenched, refrigeratedto F., and then double tempered at 600 F. for about 1 hour each cycle.Properties (average of two or three specimens) determined were asfollows:

Longitudinal pr0perties.Tensile strength, 303,000 p.s.i.; 0.2 percentyield strength, 269,000 p.s.i., 0.02 percent yield strength, 217,000p.s.i.; elongation, 3 percent and hardness, Rockwell C, 50.5.

Transverse properties.-Tensile strength, 319,000 p.s.i.; 0.2 percentyield strength, 281,000 p.s.i.; 0.02 percent yield strength, 236,000p.s.i.; elongation, 3 percent and hardness, Rockwell C, 53.

As has been previously noted in this disclosure, for certainapplications, such as rocket motor casings, it is not enough that amaterial possess ultrahigh strength with ductility; it must also possessWhat is commonly designated as fracture strength. Fracture strength is ameasure of a materials ability to Withstand stress in the presence of anotch, crack, or similar small flaw. Although there is no standard wayof determining tracture strength, there are several methods that areused. All of these methods are characterized by the fact that they arecomplex and involved. In all of them fracture strength usually decreaseswith increasing tensile strength or yield strength, for a givenmaterial.

Using one of the methods for determining fracture strength, it has beendemonstrated that compositions according to this invention have fracturestrengths of 200,- 000 p.s.i. for yield strengths of 240,000 p.s.i. SAE4340, one of the least notch-sensitivehigh strength steels, by the samemethod of determination, has a fnacture strength of 200,000 p.s.i. at amaximum yield strength of 215,000 p.s.i. Thus, the compositions of thisinvention possess yield strengths 25,000 p.s.i. or more greater than thebetter prior art high strength steels, and, at the same time, retainfracture strengths equivalent to those of the lower strength steels.This combined high fracture strengthhigh yield strength is unique to thematerials of the present invention.

The alloys of this invention may be used as aluminum-killed alloys. Forexample, the alloy of Example 3 was aluminum killed. Whenever B is usedin the present alloys then the alloy must be deoxidized with aluminum.In other instances, the alloys may be deoxidized with silicon.

New and useful ultrahigh-strength steel compositions capable of use asfuel containers for solid-propellant rocket motors, or for tensionmembers in aircraft, or for other high-strength applications have beendescribed. These alloys possess an unusual combination of yieldstrength, reduction in area, elongation properties, and fracturestrength wherefore many other advantages and applications for thesealloys will be apparent to those skilled in the art. It is desired todefine the invention in the following claims which are intended to beinterpreted, as to scope, in view of the description in thisspecification.

What is claimed is:

1. Ultrahigh-strength steel compositions consisting of from about 0.45to about 0.60 percent carbon, about .4 to 1.25 percent manganese, about2.0 to about 2.2 silicon, about 1.2 to about 1.8 nickel, about 0.16 toabout 0.35 vanadium, balance essentially iron.

2. Steel compositions according to claim 1 including, in addition, fromabout 0.0003 to about 0.10 percent boron.

3. Steel compositions according to claim 1 characterized by a 0.2percent olfset yield strength of at least about 290,000 p.s.i.

4. Steel compositions according to claim 1 further characterized by afracture strength of at least 200,000 p.s.i., at a 0.2 percent offsetyield strength of 240,000 1.5.1.

5. Steel compositions according to claim 2 characterized by a 0.2percent offset yield strength of at least 300,000 p.s.i.

6. Steel compositions according to claim 2 further characterized by afracture strength of at least 200,000 p.s.i., at a 0.2 percent offsetyield strength of 240,- 000 p.s.i.

References Cited by the Examiner UNITED STATES PATENTS 1,342,911 6/20Johnson l28.85 2,861,908 11/58 Mickelson 75123 FOREIGN PATENTS 782,7789/57 Great Britain.

DAVID L. RECK, Primary Examiner.

RAY K. WINDHAM, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,181,945 May 4, I96

Arthur R. Elsea It is hereby certified that error appears in the abovenumbered patent requiring correction and that the said Letters Patentshould read as corrected below.

Column 4, line 6, for "0010" read 0.01

Signed and sealed this 26th day of October 1965,v

(SEAL) Allest:

ERNEST W. SWIDER EDWARD J. BRENNER Altesting Officer Commissioner ofPatents

1. ULTRAHIGH-STRENGTH STEEL COMPOSITIONS CONSISTING OF FROM ABOUT 0.45TO ABOUT 0.60 PERCENT CARBON, ABOUT .4 TO 1.25 PERCENT MANGANESE, ABOUT2.0 TO ABOUT 2.2 SILICON, ABOUT 1.2 TO ABOUT 1.8 NICKEL, ABOUT 0.16 TOABOUT 0.35 VANADIUM, BALANCE ESSENTIALLY IRON.